It has been known widely in the past that a diffractive optical element having diffraction grating rings on its surface (e.g., an aspherical lens) is capable of reducing lens aberrations such as field curvature and chromatic aberration (deviation of an image-formation point depending on wavelength). If the diffractive optical element is a diffraction grating having a cross-section in a blazed form or a fine-step-like form inscribed in a blaze, the diffractive optical element is allowed to have a diffraction efficiency in a specific order of approximately 100% with respect to a single-wavelength light.
Theoretically, a depth of the diffraction grating (blaze thickness) whose diffraction efficiency for a first-order diffracted light (hereinafter, referred to as “first-order diffraction efficiency”) with respect to a certain wavelength is 100% is given as Formula 1 below:
                    d        =                  λ                                    n              ⁡                              (                λ                )                                      -            1                                              [                  Formula          ⁢                                          ⁢          1                ]            where λ represents a wavelength, d represents a diffraction grating depth, and n(λ) represents a refractive index and is a function of wavelength.
According to Formula 1, the value of d that gives a diffraction efficiency of 100% varies as the wavelength λ varies.
A diffractive optical element 110 shown in FIG. 12 is an exemplary conventional diffractive optical element. A substrate 111 is made of a material having a refractive index of n(λ), and a blaze-like diffraction grating 112 is formed on a surface of the substrate 111.
FIG. 13 is a graph showing the wavelength-dependent variation of the first-order diffraction efficiency of the diffractive optical element 110 having a diffraction grating depth d of 0.95 μm, in which the substrate 111 is made of a cycloolefin-based resin (“ZEONEX”, produced by Zeon Corporation).
The first-order diffraction efficiency is approximately 100% with respect to a wavelength of 500 nm, whereas it is about 75% with respect to wavelengths of 400 nm and 700 nm. Thus, the variation of the diffraction efficiency with wavelength (wavelength dependence) is significant. When this diffractive optical element is applied to a lens used for imaging in a wide wavelength band (e.g., a visible light wavelength range of about 400 nm to 700 nm), unwanted diffracted light is generated, which causes flare or ghost, thereby deteriorating images or degrading MTF (modulation transfer function) properties. Particularly when diffraction gratings are formed on both surfaces of a single lens or multiple surfaces in an optical system, the generation of unwanted diffracted light becomes more significant.
A diffractive optical element 130 shown in FIG. 14 is another exemplary conventional diffractive optical element. An optical material having a refractive index and a refractive index dispersion different from those of a substrate 131 is applied or cemented as a protective film 133 on a surface of the substrate 131 on which a diffraction grating 132 is formed, whereby the generation of unwanted diffracted light can be suppressed. This diffractive optical element is disclosed more specifically in the documents below.
Patent document 1 discloses an example in which the wavelength dependence of the diffraction efficiency is reduced by setting a refractive index of a substrate on which a diffraction grating is formed, and a refractive index of a protective film formed to cover the diffraction grating, to specific conditions. Patent document 2 discloses an example in which the wavelength dependence of the MTF properties is reduced under the same refractive index conditions as those disclosed in the Patent document 1.
Patent document 3 discloses an exemplary case in which the wavelength dependence of the diffraction efficiency is reduced by using materials obtained by combining a resin, glass, etc. satisfying certain refractive index conditions as materials for a substrate and a protective film.
Patent document 4 discloses that a similar effect can be achieved by using an energy curable resin containing a fluorene derivative.
Materials used in the diffractive optical element 130 are classified roughly into resins and glass. Further, refractive index conditions of members of the diffractive optical element 130 are similar to one another basically. Here, in the case where an optical material as the protective film 133 is applied or cemented onto the substrate 131 on which the diffraction grating 132 is formed, a diffraction grating depth d′ that gives a first-order diffraction efficiency of 100% is given as Formula 2 shown below:
                              d          ′                =                  λ                                                                n                ⁢                                                                  ⁢                1                ⁢                                  (                  λ                  )                                            -                              n                ⁢                                                                  ⁢                2                ⁢                                  (                  λ                  )                                                                                                    [                  Formula          ⁢                                          ⁢          2                ]            where n1(λ) represents a refractive index of a material for forming the substrate, n2(λ) is a refractive index of a material for forming the protective film, and both of n1(λ) and n2(λ) are functions of wavelength.
If the value of the right-hand side of Formula 2 becomes constant in a certain wavelength band, this means that there is no wavelength dependence of the diffraction efficiency in the foregoing wavelength band. In order to satisfy this condition and decrease the diffraction grating depth d′, an increase in the value of |n1(λ)−n2(λ)| is desired; that is, it is desired to form the substrate and the protective film with a combination of a material having a high refractive index and a low dispersion and a material having a low refractive index and a high dispersion. With this configuration, the diffraction grating depth d′ is made greater than the diffraction grating depth d of Formula 1.
FIG. 15 is a graph showing the wavelength-dependent variation of the first-order diffraction efficiency of the diffractive optical element 130 having a diffraction grating depth d′ of 84 μm, in which the substrate 131 is made of a polycarbonate resin (“Iupilon”, produced by Mitsubishi Engineering-Plastics Corporation) and the protective film 133 is made of a polystyrene-based resin produced by TOYO STYRENE Co., Ltd. As shown in FIG. 15, although the diffractive optical element 130 can achieve a reduced wavelength dependence of the diffraction efficiency compared with the diffractive optical element 110, the diffraction grating depth d′ has to be increased.
Meanwhile, some methods have been proposed as methods for manufacturing these diffractive optical elements. For example, Patent document 5 and Patent document 6 disclose manufacturing methods in which, in order to remove air bubbles that become a cause of unwanted diffracted light, an optical material to be applied is limited to one having a low viscosity, or the material is vibrated after application.    Patent document 1: JP 9 (1997)-127321 A    Patent document 2: JP 3 (1991)-191319 A    Patent document 3: JP 10 (1998)-268116 A    Patent document 4: JP 11 (1999)-287904 A    Patent document 5: JP 2001-249208 A    Patent document 6: JP 2001-235608 A
However, the diffractive optical element shown in FIG. 12 has a drawback in that, because of the wavelength dependence of the diffraction efficiency, unwanted diffracted light is generated in a certain wide wavelength band. Further, the diffractive optical element shown in FIG. 14 is capable of reducing the wavelength dependence of the diffraction efficiency and suppressing the generation of unwanted diffracted light, but has the following drawbacks: if glass is used as a material for the diffractive optical element, the molding is difficult, whereas if a resin is used, the diffraction efficiency and the wavelength dependence of the same become easily influenced by temperature variation; and the diffraction grating depth has to be increased since the materials are limited. As the diffraction grating depth is increased, there is an increased possibility that the processing of a mold for producing the diffraction grating would become difficult. Moreover, materials other than glass and resins have difficulty in maintaining transparency.